1. Field of the Invention
The present invention relates to a composition useful for forming an optically-anisotropic layer, to a retardation plate produced with the composition, and to a method for producing the retardation plate.
2. Background Art
A liquid-crystal display device generally comprises a liquid-crystal cell sandwiched between a first polarizer and a second polarizer, in which the liquid-crystal cell has a liquid-crystal layer containing a rod-shaped liquid-crystal compound between a pair of substrates. In case where the retardation to occur inside the rod-shaped liquid-crystal compound-containing liquid-crystal cell is to be canceled by an optical compensatory sheet that has an optically-anisotropic layer formed of a discotic liquid-crystal compound (e.g., 2,3,6,7,10,11-hexa{4-(4-acryloyloxyhexyloxy)benzoyloxy}triphenylene) (for example, as in JP-A-8-50206), it is impossible to simultaneously cancel the retardation for the wavelengths of all light since the wavelength dispersion of the rod-shaped liquid-crystal compound differs from that of the discotic liquid-crystal compound, therefore often resulting in discoloration (for example, failure in black coloration).
A benzene compound tri-substituted with a heterocyclic group is reported (Molecular Crystals and Liquid Crystals, 2001, Vol. 370, p. 391). It is not easy to attain low wavelength dispersion by the use of the compound, and a compound having a smaller wavelength dispersion [having a smaller value of Re (short wavelength (e.g., 450 nm))/Re (long wavelength (e.g., 650 nm))] is desired.
The retardation Re(λ) of a retardation plate must be determined in accordance with the optical properties of the liquid-crystal cell that the retardation plate is to compensate. The retardation (Δnd) is a product of the refractive anisotropy (Δn) of an optically-anisotropic layer and the thickness (d) of the optically-anisotropic layer; and an optically-anisotropic layer having a large refractive anisotropy (Δn) can compensate a liquid-crystal cell even though the thickness (d) of the layer is small. Regarding a retardation plate where the liquid crystal is aligned and fixed, the retardation Re of the plate varies depending on the tilt angle (mean tilt angle) of the aligned liquid crystal in the plate, and therefore the tilt angle in the retardation plate of the type must be controlled.
However, it is difficult to control the tilt angle of a heterocyclic group-substituted, tri-substituted benzene-type discotic liquid-crystal compound, and in particular, it is difficult to hybrid-align the compound at a low tilt angle, and therefore, an alignment controller capable of lowering the tilt angle of a discotic liquid-crystal compound to a desired angle is desired.
On the other hand, examples of adding an alignment controller or an alignment promoter to a liquid-crystal compound are disclosed, but the effect of the controller or the promoter for a tri-substituted benzene-type discotic liquid-crystal compound is not clarified, and the related art is unsatisfactory for controlling the tilt angle of the compound to a desired angle (JP-A-2002-129162).
Regarding tilt angle control, it is desired that the tilt angle change is small even though the temperature in polymerization varies, but the temperature dependence of tilt angle is great and it is desired to solve the problem.